The present invention relates to a system for evacuating from oil wells sand, paraffin, water and other non-petroleum by-products of oil production, thus enhancing such production, while at the same time efficiently and economically removing the oil from the well.
When the drilling of an oil well is first completed, it is usually expected that the petroleum and other fluid in the well will flow to the surface by the natural reservoir pressure. But in most wells, the natural pressure is not sufficient to "lift" the oil to the surface. For example, of all oil wells recently completed, approximately 90% were placed on "artificial lift" systems which bring the oil to the surface by means other than natural formation pressure. Furthermore, all of the natural lift wells, at sometime during their economic life, will diminish in natural pressure to the point where artificial lift is required to raise the fluid to the surface in order to obtain the maximum recovery of oil to the producer.
Under artificial lift production, a well will eventually reach a point where it experiences relatively low production rates. For example, in Calif., which is one of the leading oil producing states in the country, the average barrel per day production is approximately twenty-two barrels per well. In Tex., this rate is approximately fourteen, while in Okla. the same rate is only approximately five barrels per day per well. Thus, a substantial percentage of the oil produced in the United States comes from marginally producing and extensively depleted oil wells. Therefore, the production of such oil in the most efficient and economic manner is a major objective of the oil well owners or lease holders.
At the same time, extensively depleted oil wells suffer from a number of obstacles which make it difficult to efficiently and economically lift the oil. For example, oil-borne sand and other solid, non-petroleum matter frequently settles in the well bore, thus inhibiting the natural flow oil from the surrounding subterranean formations into the well bore.
Many important oil reservoirs are in sand or a sand-bearing formation. The sand, which is usually an extremely fine material having the consistency of flour, is carried by the oil in the direction of and into the well bore as oil is removed. With conventional oil recovery techniques, production always decreases with time as accumulations of sand are deposited in and around the well bore. Of all oil wells which are capped or closed, a large percentage of the closures are due to a high accumulation of sand in the well, which makes production uneconomical. In order to increase production of the oil well, the sand must be bailed out or otherwise removed from the oil well bore. However, even removal of sand from the bore itself will not restore the well to its original production because of the surrounding accumulations of sand.
By far, the predominant means for accomplishing the artificial lift of oil is the "sucker rod" pumping system. The sucker rod system accounts for approximately 90% of all artificial lift wells in the United States. This system comprises a mechanical pump which is lowered into the well at the bottom of a string of solid, rigid sucker rods, each being approximately 30 feet in length. The sucker rod and pump combination are contained within a tubing which also extends from the bottom of the well to the surface. The sucker rod string is attached to a polished rod at the surface which passes through a stuffing box and is attached to the pumping unit. The pumping unit produces the necessary reciprocating motion to actuate the sucker rods and sub-surface pump. The pumping unit generally comprises a walking beam and a counterweight for counter-balancing the weight of the rods and pump. The walking beam produces an up and down motion on the sucker rods thus pumping the fluid to the surface. In order to install the rods and sub-surface pump, a large tower is required to be placed over the wellhead for the sequential attachment of the long rods to one another. After the pumping system is in place, the tower is removed.
A severe disadvantage of the sucker rod pumping system is that it is unable to remove the sand or other solids from the well. Furthermore, in order to bail the sand, the sucker rod string and tubing must be completely removed from the oil well by the use of a production rig and crew. This process may take from two to three days and is quite expensive. The frequency with which this process is required will determine whether oil can be economically produced from a certain well. In one instance, sand had to be bailed out from a well every two days. If the oil cannot be economically produced, the well is simply capped off.
In order to reduce the sand problem, some oil wells are provided with a gravel pack surrounding the casing at the bottom of the well in order to filter the sand while permitting the crude oil to pass into the casing and be pumped to the surface. However, the sand accumulation often becomes so severe that even this method is not successful. Under such circumstances, the casing of the well must be removed, as well as the complete sucker rod system, at an even greater expense.
Moreover, the crude oil found in some parts of the United States contains a high percentage of paraffin. Another severe disadvantage of the sucker rod pumping system is that paraffin deposits in the tubing and flow lines around the sucker rods are a source of considerable trouble and expense. Again, in order to eliminate these deposits, the entire sucker rod string and tubing must be removed from the well.
Another disadvantage of the sucker rod system is that the contact by the tubing and sucker rods against the casing of the oil well can cause it to deteriorate. This is particularly a problem in "dogleg" wells in which the bore deviates from the vertical at some point by a significant amount. Such bends in the well bore are quite common and, for example, permit an on-shore well to tap an off-shore deposit. If a hole in the casing is formed then the production of the well must be discontinued while it is repaired at great inconvenience and expense, otherwise the oil will contaminate the surrounding formation which may include fresh water. Furthermore, with a hole in the casing other stimulation methods, such as steam injection, cannot be utilized.
The configuration of the sucker rod system, including its string of attached solid rods, and the requirement of a tower for installation and removal greatly complicates and makes uneconomical the use of such a system in marginally producing wells. Furthermore, the cost of the system can easily amount to $100,000, and over 50% of the sucker rod pumping systems in the United States experience some sort of failure each year.